This invention is related to apparatus for controlling the direction of the molten metal flow within a continuous caster tundish, and more particularly, it relates to providing a molten metal flow pattern to enhance inclusion float out and improve the microcleanliness of a continuous cast steel product.
A tundish is a large tub like vessel located between a continuous caster mold and the ladle used to deliver liquid steel to the caster. The tundish is designed to hold a reservoir of liquid steel which flows from the tundish into the caster mold to form a product. During the transfer of molten metal to the tundish, via a shroud extending from the ladle, the incoming molten metal stream rebounds upward from the tundish floor and creates a turbulent boiling action which breaks up the slag cover on the surface of the bath, entrains slag cover particles within the steel, and exposes the steel to the atmosphere.
Applicants' U.S. Pat. No. 5,169,591 overcomes such turbulence and slag entrainment problems through the use of an impact pad shaped to reverse the direction of the fluid flow generated by the incoming ladle stream. The impact pad includes a base and a sidewall extending in an upward direction along the periphery of the base. The ladle stream impacts upon the base and generates a radiating fluid flow toward the sidewall, and the sidewall includes an undercut extending along its inside surface, and shaped to receive and reverse the direction of the radiating fluid flow back to ward the incoming ladle stream. The reversed fluid flow dissipates the energy of the fluid flow leaving the impact pad and reduces surface turbulence within the tundish. The reversed fluid flow also increases the likelihood of collisions between inclusions, and promotes coalescence and the formation of larger inclusion particles. The larger inclusion particles float out more rapidly due to their higher buoyancy.
Research directed to fluid flow in a tundish has led to the discovery that microcleanliness can be further improved in the steel product by using additional flow control apparatus in combination with impact pads. The new flow control apparatus creates gentle upward currents to enhance inclusion float out toward the slag cover floating on the surface of the liquid steel bath. These gentle currents are directed toward the bath surface at a reduced velocity to prevent surface boil and slag cover breakup. The coalesced inclusions in the steel stream flowing from the impact pad are carded toward the slag cover where they are absorbed and improve the microcleanliness of the steel product.
Combining different pieces of tundish furniture such as an impact pad with additional flow control dams reduces the turbulent flows that create surface boil. However, it also reduces the kinetic energy level of desired liquid steel flows downstream of the tundish furniture. As a result, directional control of the downstream steel is diminished, and the flow pattern in the downstream section of the tundish is dominated by the exit flow. Regions of the tundish bath in the vicinity of the end walls and particularly in the downstream corners are bypassed by the fluid flow, resulting in undesirable stagnation. To direct the fluid flow into these stagnant areas kinetic energy must be transferred to the fluid flow from an auxiliary energy source such as inert gas bubblers or electromagnetic stirrers. Apertures extending through the flow control dam can also reduce stagnation by allowing fluid with sufficient kinetic energy to pass through the dam.
The auxiliary energy sources are positioned downstream from the flow control dam between the dam and the tundish exit nozzle. They increase the kinetic energy level and the retention time for the liquid steel in the tundish, and create gentle upward currents without generating a surface boil. The apertures extending through the dam regulate the flow volume upstream and downstream of the dam.